Fibers having a small fiber diameter can impart various excellent properties, such as a separating property, a liquid-holding capacity, a wiping property, a shading property, an insulating property, or flexibility, to a nonwoven fabric, and therefore, it is preferable that fibers which form a nonwoven fabric have a small fiber diameter. As a process for manufacturing such fibers having a small fiber diameter, electrospinning is known. In this process, a spinning liquid is extruded from a nozzle, and at the same time, an electrical field is applied to the extruded spinning liquid to thereby draw the spinning liquid and thin the diameter of the spinning liquid, and fibers are directly collected on a fibers collection means to form a nonwoven fabric. According to the electrospinning, a nonwoven fabric consisting of fibers having an average fiber diameter of 1 μm or less can be produced. It is necessary in the electrospinning that a high voltage should be applied to the nozzle or the fibers collection means, to apply an electrical field to the spinning liquid, and therefore, a complicated apparatus is needed and the electrospinning wastes energy.
To solve these problems, patent literature 1 proposes “an apparatus for forming a non-woven mat of nanofibers by using a pressurized gas stream includes parallel, spaced apart first (12), second (22), and third (32) members, each having a supply end (14, 24, 34) and an opposing exit end (16, 26, 36). The second member (22) is adjacent to the first member (12). The exit end (26) of the second member (22) extends beyond the exit end (16) of the first member (12). The first (12) and second (22) members define a first supply slit (18). The third member (32) is located adjacent to the first member (12) on the opposite side of the first member (12) from the second member (22). The first (12) and third (32) members define a first gas slit (38), and the exit ends (16, 26, 36) of the first (12), second (22) and third (32) members define a gas jet space (20). A method for forming a nonwoven mat of nanofibers by using a pressurized gas stream is also included.”, as shown in FIG. 2. This apparatus does not require the application of a high voltage, and therefore, can solve the problems. However, because flat-shaped first, second, and third members are arranged parallel to each other in the apparatus, and the pressurized gas stream is applied to a sheet-like spinning liquid, it is considered that the spinning liquid is difficult to have a fibrous form and the nonwoven fabric contains a lot of droplets, and that, if fibers can be obtained, the diameter of the fibers would become thick.
As a similar spinning apparatus, patent literature 2 proposes “an apparatus for forming nanofibers by using a pressurized gas stream comprising a center tube, a first supply tube that is positioned concentrically around and apart from the center tube, a middle gas tube positioned concentrically around and apart from the first supply tube, and a second supply tube positioned concentrically around and apart from the middle gas tube, wherein the center tube and first supply tube form a first annular column, the middle gas tube and the first supply tube form a second annular column, the middle gas tube and second supply tube form a third annular column, and the tubes are positioned so that first and second gas jet spaces are created between the lower ends of the center tube and first supply tube, and the middle gas tube and second supply tube, respectively”. This apparatus also does not require the application of a high voltage, and can solve the problems. However, because the pressurized gas stream is applied to a spinning liquid annularly extruded, spinning cannot be stably performed, and the spinning liquid is difficult to have a fibrous form and the nonwoven fabric contains a lot of droplets.